Stull had just taken off on his first test flight of a new design shortly after 3 pm on Wednesday when the accident occurred. According to a witness, he had climbed to about 50 feet when the aircraft stalled, flipped, and fell to the ground. Stull died instantly.

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The aircraft Stull was flying reportedly featured a large, boxlike tail, unlike any of his previous designs. Stull was always trying new configurations of airplane wing and fuselage, constantly tweaking and modifying his aircraft, and would regularly take them apart and start new ones from scratch. This meant that he was frequently flying untested designs whose flying characteristics became clear only once he was in the air.

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Stull understood the risks he was taking. "I have had many near-death experiences in my life. I am a thrill seeker," he told Popular Mechanics earlier this year. "I used to be a white-water kayaker, paddling steep, flooded creeks in Oregon. We used to say, if we don't almost die, we aren't having fun. I have no fear of death, but I fear being seriously injured."

Experimental aviation was an addictive thrill for Stull, who continued to fly compulsively despite numerous scares. "I have had seven engine failures over the years where I had to glide and land at places other than airports," he said. "I had damage in two of those landings but was never injured. I fly about three days a week, all year, which is way, way more than most."

Among the dangers of his hobby was flying over rugged rangeland near his hometown of Christoval in Central Texas. "Flying an ultralight over hostile terrain takes guts," he wrote. "You never know when the engine might die. But even more scary is being the test pilot, doing the first stalls to check the center of gravity, accelerated stalls, steep banked turns, diving to check for high-speed flutter, pulling g's to verify the airframe structure. I gradually learned test-piloting skills."

We may never know exactly what caused his final, fatal mishap. Because Stull's plane weighed less than 254 pounds and had room only for the pilot, the Federal Aviation Administration deems it an ultralight. Classifed as such, the National Transportation Safety Board will not conduct an investigation. The government's attitude is, essentially, that if you want to fly around in a plane small enough that it poses a threat only to yourself, then go ahead.

It's a dangerous kind of freedom, but one that Mark Stull embraced to the fullest.